Focus on APS Topical Groups

Hadronic Physicists Find a Home

By Lauren Schenkman

In textbook diagrams, the proton can look deceptively simple, a tiny spherical sun holding the electron in its orbit. But furled inside the proton is a mysterious congeries of quarks, antiquarks, and gluons that obey laws still muddy to physicists. Known as quantum chromodynamics, these laws are fertile ground for new discoveries, including the remarkable property of “asymptotic freedom” that earned David Gross, Frank Wilczek, and H. David Politzer the 2004 Nobel Prize. But quantum chromodynamics is far from completely understood.

“We are still struggling with much of the basic theory,” said Stan Brodsky, a theorist at Stanford University and SLAC National Accelerator Laboratory and the rising chair of the APS Topical Group on Hadronic Physics. “It’s so complicated.”

As a consequence, some of the world’s most powerful particle accelerators are devoted to peering into the proton. Brookhaven National Lab’s Relativistic Heavy Ion Collider smashes beams of gold ions, melting the protons and neutrons and freeing their quarks and gluons in a plasma that’s hotter than the sun and lasts just a few billionths of a second. Meanwhile, Thomas Jefferson National Accelerator Facility’s accelerator fires an electron beam at a proton target, probing the three-dimensional quark structure for explanations of the proton’s macroscopic properties, such as mass, spin, and magnetic moment. Other high-profile facilities, such as the Large Hadron Collider and Fermilab, also host important hadronic physics projects.

Hadronic physics plays a role in medicine and energy; proton and pion beams fight tumors, and a greater grasp of protons would improve understanding of nuclear fuels. Meanwhile, physicists working on experiments often come up with new technologies that find their way into industry, in the form of better photomultipliers or detectors that can be used to improve Magnetic Resonance Imaging.

Despite this field’s importance and prominence, hadronic physicists lacked a community within APS until 2002. Many hadronic physicists were members of the Division of Particles and Fields or the Division of Nuclear Physics, but their particular interests fell through the cracks between these two areas. In early 2000, led by Eric Swanson, Ted Barnes, Alex Dzierba and James Bjorken, hadronic physicists began pushing to form a cohesive community within APS, achieving official status in 2002.

“Our goals were to raise the visibility of hadronic physics,” Swanson explained. “The field had (and still has) the problem that it is a part of particle physics that is not pursuing the Higgs or the next Standard Model, so it is missing a natural home. It also tends to be funded by Nuclear Physics, which heightens the confusion.”

According to Winston Roberts, the group’s current chair and a theorist at Florida State University, one of the GHP’s goals is to provide hadronic physicists with “a forum to discuss things they’re interested in.”

To that end, the group held its third biennial conference on hadronic physics just days before the 2009 April Meeting in Denver. It was the largest yet, featuring more than 90 participants and 80 talks on everything from jet physics to lattice quantum chromodynamics to heavy-ion physics.

“I think the meetings we organize do generate an atmosphere in which new collaborations can get formed,” Roberts said. Members can look forward to a strong presence at next year’s April Meeting in February 2010, when the GHP will present two invited sessions on the latest advances.

The GHP has also played an important role in nominating deserving hadronic physicists for APS Fellowship, ensuring that they’re not lost in the particle physics and nuclear physics crowd. But that role would benefit from an increased membership. While the group has grown to include about 400 physicists, Roberts hopes to see the community expand even further.

“The membership could be a lot larger than it currently is,” he said. “There are people who are members of [the Division of Particles and Fields and the Division of Nuclear Physics] who should probably consider joining GHP.”

“QCD is so important, it deserves a central domain,” Brodsky agreed. “This is really the natural place for presenting the latest work.”